A 200-400 metre wide asteroid first discovered by the Wide-field Infrared Survey Explorer (WISE) has been found by astronomers at Armagh Observatory to have been following Earth in its orbit for at least a quarter of a million years, and could represent a theorized but not-yet-seen population of asteroid.

2010 SO16's average distance from the Sun is identical to that of the Earth, unlike typical near Earth asteroids (NEAs) that loop through the Solar System on highly elliptical orbits. It is also the largest of Earth’s so-called "horseshoe" asteroids, which mimic closely the orbital motion of our planet around the Sun, but as seen from Earth, appear to slowly trace out a horseshoe shape in space.

"The asteroid takes 175 years to move from one end of the horseshoe to the next," Apostolos “Tolis” Christou tells Astronomy Now. "For example, SO16 was near the leading edge of the horseshoe in the 1830s and before that in the 1660s. Also, its elongation from the Sun (and thus its visibility) is maximum near the ends of the horseshoe."

Even though its orbit is remarkably similar to Earth’s, “this asteroid is terraphobic”, says Christou. “It keeps well away from the Earth. So well, in fact, that it has likely been in this orbit for several hundred thousand years, never coming closer to our planet than 50 times the distance to the Moon”. This is where it is now, near the end of the horseshoe trailing the Earth.

Christou and colleague David Asher wanted to find out how stable its orbit is and how long it has been locked in this configuration. To eliminate uncertainty in an asteroid's orbit usually requires months or years' worth of observations, but the team sidestepped this by using computer simulations to investigate every possible orbit that it could conceivably occupy, by following the evolution of the orbits for two million years into the past and future. The majority of the asteroids remained in the observed horseshoe state for between 200,000 and 500,000 years.

But, says Christou, if it's in a stable orbit, it is difficult to put it there by a random process. "It could be a piece of the Moon that wandered into the horseshoe a million years ago or so," he speculates. "The problem with that hypothesis is that most lunar impact ejecta are usually smaller than the size of the object. It could also have been a run-of-the-mill NEO that drifted in there through the vaunted Yarkovsky effect [which is caused by heat emitted from a small rotating body, carrying momentum with it] but that would mean it was at some point close to the Earth's orbit but not protected from close encounters with the Earth by the horseshoe mechanism. Hence its orbit would have quickly become un-Earth-like."

Instead, Christou and Asher propose that SO16 is "leakage" from a population of Earth Trojans, a breed of asteroid located at triangular equilibrium points 60 degrees ahead of and behind the Earth in its orbit that have only ever been theorized and never yet observed due to their proximity to the Sun in the sky. Similar populations of asteroids are known at Jupiter and Neptune (see our news story Tojan asteroid found in Neptune's gravity 'void' here).

"This is the simplest process-wise, although one has to show that there is such a reservoir of objects," says Christou. "That's the way it usually works; one question answered, a million spawned!"

The Faulkes Telescope project is already being used to refine the asteroid's orbit, and further investigation to uncover the asteroid's physical properties will enable the astronomers to reach their conclusion regarding its origin.